1. Field of the Invention
The invention relates generally to the field of seismic exploration. More particularly, the invention relates to methods for acquiring marine seismic data using selected arrangements of sources and receivers.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth's surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in the ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations. Acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as “bed boundaries”. At an acoustic impedance boundary, some of the seismic energy is reflected back toward the earth's surface, where it may be detected by one or more of the seismic sensors deployed on the surface. Other portions of the energy are refracted and continue propagating in a generally downward direction until another impedance boundary is reached. Seismic signal processing known in the art has as an objective the determination of the depths and geographic locations of bed boundaries below the earth's surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the acoustic impedance boundaries and back to the sensors at the surface.
Seismic surveying (marine seismic surveying) is performed in the ocean to determine the structure of earth formations below the sea bed. Marine seismic surveying known in the art includes having a vessel tow one or more seismic energy sources, and the same or a different vessel tow one or more “streamers”, which are arrays of seismic sensors forming part of or otherwise affixed to a cable. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other, in a pattern selected to enable relatively complete determination of geologic structures in three dimensions.
The signals detected by the seismic sensors at the earth's surface include components of seismic energy reflected at the bed boundaries, as previously explained. In addition, both coherent noise (noise which has a determinable pattern, such as may be caused by a ship propeller) and incoherent (random) noise may be present. The presence of such noise in the signals received by the seismic sensors reduces the signal-to-noise ratio (“SNR”) of the seismic signals of interest. An objective of seismologists, therefore, is to seek methods of eliminating the effects of noise on the signals detected by the sensors without appreciably reducing the true seismic signal component of the detected signals.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality seismic representation of a particular subsurface structure include using multiple actuations of the seismic source (multiple “firings” or “shots”) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or “stacking” such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise.
U.S. Pat. No. 5,818,795, which is assigned to the assignee of the present invention provides a detailed summary of prior art methods and systems addressing the problem of noise elimination in seismic signals, and discloses a method of reducing the effect of “burst” noise in seismic signal recordings without eliminating actual reflection data.
U.S. Pat. No. 5,761,152, which is assigned to the assignee of the present invention describes a method and system for marine seismic surveying. The method disclosed in the '152 patent includes increasing the fold (number of recorded reflections from a same reflector), and hence the signal-to-noise ratio of seismic signals, without incurring the problems of drag, entanglement, complicated deck handling, and decreased signal-to-noise ratio associated with increased streamer length, increased number of streamers, and increased distance between streamers. Source and streamer “offsets”, and time of firing of lead and trailing vessel sources in a time delay sequence are optimized to increase the fold while avoiding any influence by the seismic signals resulting from the source of one vessel on the seismic signals resulting from the source of the other vessel.